Liquid crystal devices are more and more used in many different applications. Examples are liquid crystal displays (LCD) and optical films, in particular polarizing films and retardation films, as well as security devices for preventing forgery, counterfeiting and copying.
The successful functioning and performance of a liquid crystal device relies on the ability of the liquid crystal molecules within that device to adopt and maintain an alignment imposed upon them. Alignment of the liquid crystal molecules is achieved by use of an alignment layer which defines a direction of orientation for the liquid crystal molecules with the result that the longitudinal axes of the molecules become aligned with the direction of orientation defined by the alignment layer. In addition to this directional alignment, for some applications the alignment layer should also be able to impart to the liquid crystal molecules an angle of tilt so that the molecules align themselves at an angle out of the surface of the alignment layer.
A well known method for preparing alignment layers is a rubbing treatment wherein a high molecular resin film such as polyimide is rubbed in a single direction with a cloth. The liquid crystal molecules adjacent to the rubbed surface are aligned in the rubbing direction. However, alignment films formed by rubbing have some disadvantages like dust generation and scratches, which occur during the rubbing process. In addition, rubbing methods are not adequate for the production of structured layers, i.e layers having small areas with different alignment directions.
These problems can be solved using liquid-crystal alignment control processes other than rubbing such as oblique deposition, photolithographic, Langmuir Blodgett film, ion irradiation, high velocity fluid jet and other processes. However, most of these processes are not practical for processing large-area substrates.
Other methods developed for the alignment of liquid crystals are alignment layers made by photo-orientation methods (usually using linearly polarized light), and especially well suited are linearly photo-polymerized (LPP) alignment layers, also known as photo-oriented polymer networks (PPN). Such methods are for instance disclosed in U.S. Pat. No. 5,389,698, U.S. Pat. No. 5,838,407, U.S. Pat. No. 5,602,661, U.S. Pat. No. 6,160,597, U.S. Pat. No. 6,369,869, U.S. Pat. No. 6,717,644, U.S. Pat. No. 6,215,539, U.S. Pat. No. 6,300,991, and U.S. Pat. No. 6,608,661.
These methods allow the generation of homogeneous alignment of liquid crystals. In the LPP process, which is a non-contact technique, alignment films similar to those obtained by rubbing can be obtained with high reproducibility by irradiating a photosensitive film on a large substrate area with polarized light. In addition, it is possible to vary the direction of orientation and the azimuthal and polar tilt angle within the photoreactive layer by controlling the direction of the irradiation of the linearly polarized light. By selectively irradiating specific regions of the photoreactive material, it is possible to align very specific regions of the layer and thus to provide alignment areas having different orientation which gives rise to structured alignment layer as described for example in Jpn. J. Appl. Phys., 31 (1992), 2155-64 (Schadt et al.).